Selenium (Se) radioactive wastes can be disposed through stabilization/solidification (S/S) based on the cementitious matrix on hydration products, where hydrocalumite (Ca₂Al-LDH) is expected to play an important role in the retention of SeO₄²⁻. Natural organic matters (NOMs) are known to be a risk to affect the transportation and mobility of undesirable chemical species in the pedosphere which receives the low level radioactive wastes (LLW). In the present work, five amino acids were selected as the simplified models of NOMs in the pedosphere to explore their effects on the stability of Ca₂Al-LDH after immobilized SeO₄²⁻ under alkaline conditions. As the loading amount of amino acids on Ca₂Al-LDH increasing, release of SeO₄²⁻ was enhanced in HGly, H₂Asp, and H₂Cys series, while no enhancement was observed in HPhe and HTrp series. Density functional theory (DFT) calculation predicted ion-exchange of amino acids and CO₃²⁻ with SeO₄²⁻ in a unit cell of LDH model. The intercalation of Asp²⁻ and CO₃²⁻ caused 003 peaks in XRD sharper and d₀₀₃ decreased from 8.15 Å to 7.70 Å which is assigned to Ca₂Al-LDH(Asp, CO₃). In H₂Cys series, the 003 peaks were kept broad and SeO₄²⁻ was still relatively maintained in LDH which was caused by the lower amounts of intercalated CO₃²⁻ in the presence of H₂Cys. Amino acids in the interlayer of Ca₂Al-LDH have several possible configurations, where the most stable one is prone to be in a horizontal direction through hydrogen bonds and Ca–O chemical bonds. This provides an insight on the stability of selenate immobilized in hydrocalumite, which can be produced in cement disposing in the pedosphere for a long term of burying. Not only carbonate but also small molecular organic matters like amino acids possibly give environmental impact on the mobility of low level anionic radionuclides in LDH.

The negative effect of carbonate on the rate and extent of bioreduction of aqueous U(VI) has been commonly reported. The solution pH is a key chemical factor controlling U(VI)ₐq species and the Gibbs free energy of reaction. Therefore, it is interesting to study whether the negative effect can be diminished under specific pH conditions. Experiments were conducted using Shewanella putrefaciens under anaerobic conditions with varying pH values (4–9) and bicarbonate concentrations ([CO32−]T, 0–50 mmol/L). The results showed a clear correlation between the pH-bioreduction edges of U(VI)ₐq and the [CO32−]T. The specific pH at which the maximum bioreduction occurred (pHₘbᵣ) shifted from slightly basic to acidic pH (∼7.5–∼6.0) as the [CO32−]T increased (2–50 mmol/L). At [CO32−]T = 0, however, no pHₘbᵣ was observed in terms of increasing bioreduction with pH (∼100%, pH > 7). In the presence of [CO32−]T, significant bioreduction was observed at pHₘbᵣ (∼100% at 2–30 mmol/L [CO32−]T, 93.7% at 50 mmol/L [CO32−]T), which is in contrast to the previously reported infeasibility of bioreduction at high [CO32−]T. The pH-bioreduction edges were almost comparable to the pH-biosorption edges of U(VI)ₐq on heat-killed cells, revealing that biosorption is favorable for bioreduction. The end product of U(VI)ₐq bioreduction was characterized as insoluble nanobiogenic uraninite by HRTEM. The redox potentials of the master complex species of U(VI)ₐq, such as (UO2)4(OH)7+, (UO2)2CO3(OH)3−, and UO2(CO3)34−, were calculated to obtain insights into the thermodynamic reduction mechanism. The observed dynamic role of pH in bioreduction suggests the potential for bioremediation of uranium-contaminated groundwater containing high carbonate concentrations.